Title: Carbonation of Fly Ash Concrete

Author(s): M. D. A. Thomas, J. D. Matthews, and C. A. Haynes

Publication: Symposium Paper

Volume: 192

Issue:

Appears on pages(s): 539-556

Keywords: carbonation; curing; fly ash; serviceability

DOI: 10.14359/5770

Date: 4/1/2000

Abstract:
This paper reports data from a study on the effect of curing and strength grade on the carbonation of concrete with and without fly ash. Three series of concrete mixtures were cast with strength grades of 25, 35, and 45 Mpa, within each series the level of fly ash ranged from 0 to 50% by mass of total cementitious material. Concrete specimens were cured in a variety ways prior to exposure in one of three conditions; these were (I) indoors in laboratory air at 20 degrees C and 65% R.H. in a CO2-rich atmosphere (10% CO2). The paper presents data for specimens up to ten years of age. The results show that the carbonation of concrete with 15% to 30% fly ash is similar to or slightly higher than equivalent strength concrete without fly ash. Concrete containing 50% fly ash carbonated at a significantly faster rate than control specimens of the same grade and the differences were particularly marked for poorly cured concretes. These results merely reinforce the need to pay particular attention to curing when using high levels of fly ash and should not become a barrier to using concrete with high levels of fly ash and should not become a barrier to using concrete with high levels of fly ash. The behavior of concretes stored at 10% CO2 was inconsistent. In some cases, the carbonation front penetrated a few millimeters into the concrete during the first 3 to 7 days and there was little further penetration observed with subsequent storage to 56 days. In other cases, the carbonation depth increased proportionally with the square root of time as expected. Consequently, the results from these accelerated tests did not show a reliable correlation with the long-term data from natural exposure. Simple service-life estimates show that 35-Mpa and 45-Mpa concrete (grades based on cube strengths) with up to 30% fly ash can be expected to meet the requirements for design lives of 50 years and 100 years, respectively, provided a minimum cover of 30 mm is achieved. For concrete with 50% fly ash it may be necessary to increase the design strength (e.g. by 4 Mpa) or extent the period of moist curing (e.g. from 1 to 3 days) to meet the same requirements. These estimates assume conditions where carbonation-induced corrosion is the predominant form of deterioration.